Computed Tomography (CT) colonoscopy, also known as virtual colonoscopy, is a technique for detecting polyps in the colon. Colon cancer is often preceded by the presence of a benign polyp before it becomes malignant. The size of a polyp is important because it helps predict the likelihood of malignancy. The measured size of a polyp in CT colonography images is used to determine the action a patient has to undergo after examination.
Currently, the polyp size on two-dimensional (2D) or three-dimensional (3D) colonography images may be measured manually by measuring the distance between two points placed at two locations on the polyp base visualized in an image. The CT colonography software computes the Euclidean distance between the two points. When using a 2D image to perform the measurement, e.g., on an axial or multiplanar reformatted image, each point is placed on the colon wall somewhere halfway between the black (the colon air) and the white (the colon wall) gray value. In 3D, the points are positioned directly on the visible colon surface. The polyp size is determined on the basis of the distance of the two points. A manual measurement technique for measuring the polyp size is described by P. J. Pickhardt et al. in an article entitled “Linear polyp measurement at CT colonography: In vitro and in vivo comparison of two-dimensional and three-dimensional displays”, published Radiology, vol. 236 (2005), no. 3, pages 872-878. In automatic measurements, the size of the polyp is computed based on image data segmentation results. An automatic measurement technique for measuring the polyp size is described by D. Burling et al. in an article entitled “Ct colonography: Automatic measurement of polyp diameter compared with manual assessment—an in-vivo study”, published in Clinical Radiology 62 (2007), pages 145-151.
The polyp size is visualized by a line segment defined by the two points in a manual approach or floating inside the colon wall or in the air above the polyp in an automatic approach. This way of visualizing the measured polyp size does not always show the relationship between the measurement and the measured object.
Further, in a manual approach, the measured size of the object may be less accurate. FIG. 1A schematically illustrates a manually measured polyp size according to prior art. In a top view 11, two points 101 and 102 are selected on a contour 100 which is assumed to represent a projection of the polyp on a plane perpendicular to the viewing direction. The distance D1 between the selected two points 101 and 102 is accepted as the size of the polyp. However, in a side view 12 of the polyp one can see that the distance D1 does not coincide with the size of the contour 100.